Download Complex Numbers

Lecture 1: Complex Numbers
Key Points
• A point on a two-dimensional plane (the complex plane)
• Usually denoted by z
• Can be defined either using Cartesian (x,y) or polar (r, θ)
coordinates
• Addition and scaling complex numbers is just like adding
and scaling vectors
What is a complex number?
• A complex number z is a point (or vector) on a twodimensional plane, known as the complex plane and
represented by C.
Cartesian Coordinates
• The Cartesian coordinates of z are
▫ x = Re{z}; the real part of z
▫ y = Im{Z}; the imaginary part of z
• The corresponding axes are known as the real and
imaginary axes, respectively.
Polar Coordinates
• The polar coordinates of z are
• r =|z|; the modulus, or magnitude, of z
• θ= z; the angle of z
Converting Between Coordinates
The Cartesian coordinate pair (x, y) is also equivalent to the
polar coordinate pair (r,θ), where r is the (nonnegative)
length of the vector corresponding to (x, y), and θ is the angle
of the vector relative to positive real line.
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x = r cos θ
y = r sin θ
Z = x + jy = r cos θ + j rsin θ = r (cos θ + j sin θ)
|z| = r = √(x2 + y2)
tanθ = (y/x)
θ = arctan(y/x)+ (0 or Π) (Π is added iff x is negative)
Note that 2 rad = 360° = one full revolution, and thus angles which differ by
multiples of 2 are equivalent. Usually, θ is quoted in the interval [0; 2  ) or (;  ].
Example
• Consider the complex numbers z1 and z2, where
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▫
▫
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Re{z1}= -5;
Im{z1} = 2,
|z2| = 4
 z2=/6 rad
• Your task: Plot these on the complex plane.
Example
• The modulus of z1 is given by
▫ |z1|= √(-5)2 + 22) = 5.3852
• Its angle is given by
▫ z1 = arctan(-2/5) +  = 2.7611 rad or 158.2°.
▫  was added to the result of arctan(-2/5) because the real
part of z1 is negative.
• As for z2, we have
▫ Re{z2} = 4 cos(/6) = 2√3 = 3.4641
▫ Im{z2} = 4 sin( /6) = 2
Example
• Your task: Using as little algebra as possible, repeat for
▫
▫
▫
▫
Re{z3}= 5;
Im{z3} = -2,
|z4| = 4
 z2=-/6 rad
Summation of Complex Numbers
•
The sum of two complex numbers is a complex
number:
(x1 + jy1) + (x2 + jy2) = (x1 + x2) + j(y1 + y2);
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Example, Express the following complex numbers
in the form x + jy, x, y real:
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(−3 + j)+(14 − 2j)
Since complex numbers are vectors, expressions such as cz (scaling by a
real constant c) or z1+z2 (summation) have the same meaning as in the case
of two-dimensional vectors. Clearly, summation of two complex numbers is
easiest to perform using Cartesian coordinates (i.e., real and imaginary
parts).